Fuel compositions

ABSTRACT

A fuel composition containing a gas oil base fuel, an alkyl levulinate and one or more additional components, each of which components contains one or more aromatic constituents is provided. A method of reducing the phase separation temperature of a fuel composition is described.

FIELD OF THE INVENTION

The present invention relates to fuel compositions comprising a gas oil base fuel.

BACKGROUND OF THE INVENTION

Two different fuel components can be blended to modify the properties and/or the performance, e.g. engine performance, of the resultant composition.

Diesel fuel components can include the so-called “biofuels” which derive from biological materials. Examples include levulinate esters.

Levulinate esters (esters of levulinic acid) and their preparation by reaction of the appropriate alcohol with furfuryl acetate are described in Zh. Prikl. Khim. (Leningrad) (1969) 42(4), 958-9, and in particular the methyl, ethyl, propyl, butyl, pentyl and hexyl esters.

WO-A-94/21753 discloses fuels for internal combustion engines, including both gasoline and diesel fuel, containing proportions (e.g. 1 to 90% v, 1 to 50% v, preferably 1 to 20% v) of esters of C₄₋₆ keto-carbonic acids, preferably levulinic acid, with C₁₋₂₂ alcohols. Esters with C₁₋₈ alcohols are described as being particularly suitable for inclusion in gasolines, and esters with C₉₋₂₂ alcohols are described as being particularly suitable for inclusion in diesel fuels. The examples in WO-A-94/21753 are about the inclusion of quantities of levulinate esters in gasolines, for improvement in octane numbers (RON and MON).

WO-A-03/002696 discloses a fuel composition incorporating levulinic acid, or a functional derivative thereof, with the object of providing more oxygen by volume than ethanol or traditional oxygenates such as MTBE or ETBE, giving little or no increase in fuel Reid vapour pressure and little or no effect on the flash point of the base fuel. The functional derivative is preferably an alkyl derivative, more preferably a C₁₋₁₀ alkyl derivative. Ethyl levulinate is said to be preferred, with methyl levulinate a preferred alternative. The levulinic acid or functional derivative is preferably used to form 0.1 to 5% v of the fuel.

Current commercially available compression ignition (diesel) engines tend to be optimised to run on fuels having a desired specification. Moreover, the conditions under which the engine is required to operate can affect the manner in which a fuel composition in the engine will behave. In particular, as the atmospheric temperature falls, a fuel that is a single-phase homogeneous liquid at normal temperatures may become a multiphase liquid as certain components either (i) freeze (forming solid wax) or (ii) become immiscible in the bulk liquid and form a separate liquid layer. The onset of wax formation on cooling is characterised by a change in the transparency of the fuel and the temperature at which this occurs is termed the “Cloud Point” of the fuel. If, on cooling, the Cloud Point is preceded by the formation of a separate liquid phase, the temperature at which this occurs is termed the “Phase separation temperature”. Diesel fuel specifications such as ASTM D975-02 (USA) and EN590 (Europe) include limits on Cloud Point temperature in order to ensure that diesel fuel remains fluid at the lowest anticipated service temperature and that blocking of fuel filters by wax is prevented. For trouble free operation, it is also desirable that the diesel fuel in the fuel tank remains homogeneous, since the composition of some or all of any separated liquid layers may be unsuitable as a fuel for the engine. The blending of a standard commercial diesel base fuel with other fuel components, to modify the overall fuel properties and/or performance, can therefore have an adverse impact on the performance of the blend in the engines for which it is intended.

For the above reason, it is desirable for any diesel fuel blend to have an overall specification as close as possible to that of the standard commercially available diesel base fuels for which engines tend to be optimised.

This can, however, be difficult to achieve because any additional fuel component is likely to alter the properties and performance of the base fuel. Moreover the properties of a blend, in particular its effect on low temperature performance, are not always straightforward to predict from the properties of the constituent fuels alone.

SUMMARY OF THE INVENTION

A fuel composition is provided comprising a gas oil base fuel, an alkyl levulinate and at least one additional component, each of which component contains one or more aromatic constituents.

Further, a method of reducing the phase separation temperature of a fuel composition containing a gas oil base fuel and an alkyl levulinate is provided, said method comprising increasing the level of aromatic constituents in the fuel composition.

Further a method of reducing the phase separation temperature of a fuel composition containing a gas oil base fuel and an alkyl levulinate is provided, said method comprising the steps of incorporating in the fuel composition one or more additional components, each of which components contains one or more aromatic constituents.

A method of operating a compression ignition engine and/or a vehicle using such fuel composition is provided.

Further a method of operating a heating appliance using such fuel composition is provided.

A process for the preparation of such fuel composition is also provided.

DETAILED DESCRIPTION OF THE INVENTION

It has now been found that in fuel compositions comprising a gas oil base fuel and an alkyl levulinate, the phase separation temperature of the fuel composition is dependent upon the level of aromatic constituents in the base fuel. In particular, the phase separation temperature is lowered when the level of aromatic constituents is increased. Moreover, at a particular temperature, if the level of aromatic constituents is increased, the amount of alkyl levulinate that can be incorporated in a homogeneous mixture is increased.

In accordance with one embodiment of the present invention there is provided a fuel composition comprising a gas oil base fuel, an alkyl levulinate and one or more additional components, each of which components contains one or more aromatic constituents.

In accordance with another embodiment of the present invention there is also provided a method of reducing the phase separation temperature of a fuel composition comprising a gas oil base fuel and an alkyl levulinate, which method comprises increasing the level of aromatic constituents in the fuel composition, preferably by incorporating in the fuel composition one or more additional components, each of which components contains one or more aromatic constituents.

In accordance with yet another embodiment of the present invention there is further provided use in a fuel composition comprising a gas oil base fuel and an alkyl levulinate of one or more additional components, each of which components contains one or more aromatic constituents, for the purpose of reducing the phase separation temperature of the fuel composition

In accordance with another embodiment of the present invention there is further provided a method of operating a compression ignition engine and/or a vehicle which is powered by such an engine, which method involves introducing into a combustion chamber of the engine a fuel composition according to the present invention.

In accordance with yet another embodiment of the present invention there is further provided a method of operating a heating appliance provided with a burner, which method comprises supplying to said burner a fuel composition according to the present invention.

In accordance with yet another embodiment of the present invention there is further provided a process for the preparation of a fuel composition which process involves blending a gas oil base fuel, an alkyl levulinate and one or more additional components, each of which components contains one or more aromatic constituents.

Preferably, said alkyl levulinate is selected from C₂₋₈ alkyl levulinates, more preferably ethyl levulinate, n-propyl levulinate, n-butyl levulinate, n-pentyl levulinate, 2-hexyl levulinate, 2-ethyl hexyl levulinate, and mixtures thereof still more preferably ethyl levulinate, n-butyl levulinate and/or n-pentyl levulinate, most preferably ethyl levulinate.

Preferably, said additional components are selected from materials which are suitable to be blended with fuel compositions, such as for example (i) a refinery product stream with an aromatic content higher than that of the base fuel, or (ii) an aromatic solvent, e.g. SHELLSOL AB (available ex. Shell companies), boiling in the normal temperature range of gas oil.

Preferably, the concentration of said additional component(s) in the fuel composition accords with one or more of the following parameters:

(i) at least 1% m; (ii) at least 5% m; (iii) at least 10% m (iv) at least 15% m; (v) up to 25% m; (vi) up to 30% m; (vii) up to 40% m, (viii) up to 50% m, with ranges having features (i) and (viii), (ii) and (vii), (iii) and (vi), and (iv) and (v) respectively being progressively more preferred.

Preferably, the concentration of aromatic constituents in the fuel composition containing said additional component(s) accords with one or more of the following parameters:

(i) at least 1% m; (ii) at least 5% m; (iii) at least 10% m (iv) at least 20% m; (v) up to 30% m; (vi) up to 35% m; (vii) up to 40% m, (viii) up to 50% m, with ranges having features (i) and (viii), (ii) and (vii), (iii) and (vi), and (iv) and (v) respectively being progressively more preferred.

Preferably, said phase separation temperature of said fuel composition is reduced by at least 3° C., more preferably by at least 5° C., still more preferably by at least 10° C., and most preferably by at least 20° C.

Preferably, said phase separation temperature of said fuel composition is below −5° C., more preferably below −10° C., still more preferably below −20° C., and most preferably below −30° C.

In all aspects of the present invention, blends of two or more of the alkyl levulinates may be included in the fuel composition. In the context of the present invention, selection of the particular components of said blends and their proportions is dependent upon one or more desired characteristics of the fuel composition.

The present invention may be used to formulate fuel blends which are expected to be of particular use in modern commercially available diesel engines as alternatives to the standard diesel base fuels, for instance as commercial and legislative pressures favour the use of increasing quantities of organically derived “biofuels”.

In the context of the present invention, “use” of a fuel component in a fuel composition means incorporating the component into the composition, typically as a blend (i.e. a physical mixture) with one or more other fuel components, conveniently before the composition is introduced into an engine.

The fuel composition will typically contain a major proportion of the base fuel, such as from 50 to 99% v, preferably from 50 to 98% v, more preferably from 80 to 98% v, most preferably from 90 to 98% v. The proportions of the alkyl levulinates will be chosen to achieve the desired degree of miscibility, i.e. phase separation temperature, and may also be influenced by other properties required of the overall composition.

The fuel compositions to which the present invention relates include diesel fuels for use in automotive compression ignition engines, as well as in other types of engine such as for example marine, railroad and stationary engines, and industrial gas oils for use in heating applications (e.g. boilers).

The base fuel may itself comprise a mixture of two or more different diesel fuel components, and/or be additivated as described below.

Such diesel fuels will contain a base fuel which may typically comprise liquid hydrocarbon middle distillate gas oil(s), for instance petroleum derived gas oils. Such fuels will typically have boiling points within the usual diesel range of 150 to 400° C., depending on grade and use. They will typically have a density from 750 to 900 kg/m³, preferably from 800 to 860 kg/m³, at 15° C. (e.g. ASTM D4502 or IP 365) and a cetane number (ASTM D613) of from 35 to 80, more preferably from 40 to 75. They will typically have an initial boiling point in the range 150 to 230° C. and a final boiling point in the range 290 to 400° C. Their kinematic viscosity at 40° C. (ASTM D445) might suitably be from 1.5 to 4.5 mm²/s.

Such industrial gas oils will contain a base fuel which may comprise fuel fractions such as the kerosene or gas oil fractions obtained in traditional refinery processes, which upgrade crude petroleum feedstock to useful products. Preferably such fractions contain components having carbon numbers in the range 5 to 40, more preferably 5 to 31, yet more preferably 6 to 25, most preferably 9 to 25, and such fractions have a density at 15° C. of 650 to 1000 kg/m³, a kinematic viscosity at 20° C. of 1 to 80 mm²/s, and a boiling range of 150 to 400° C.

Optionally, non-mineral oil based fuels, such as bio-fuels or Fischer-Tropsch derived fuels, may also form or be present in the fuel composition. Such Fischer-Tropsch fuels may for example be derived from natural gas, natural gas liquids, petroleum or shale oil, petroleum or shale oil processing residues, coal or biomass.

The amount of Fischer-Tropsch derived fuel used in a diesel fuel composition may be from 0.5 to 100% v of the overall diesel fuel composition, preferably from 5 to 75% v. It may be desirable for the composition to contain 10% v or greater, more preferably 20% v or greater, still more preferably 30% v or greater, of the Fischer-Tropsch derived fuel. It is particularly preferred for the composition to contain 30 to 75% v, and particularly 30 or 70% v, of the Fischer-Tropsch derived fuel. The balance of the fuel composition is made up of one or more other fuels.

An industrial gas oil composition will preferably comprise more than 50 wt %, more preferably more than 70 wt %, of a Fischer-Tropsch derived fuel component.

Such a Fischer-Tropsch derived fuel component is any fraction of the middle distillate fuel range, which can be isolated from the (hydrocracked) Fischer-Tropsch synthesis product. Typical fractions will boil in the naphtha, kerosene or gas oil range. Preferably, a Fischer-Tropsch product boiling in the kerosene or gas oil range is used because these products are easier to handle in for example domestic environments. Such products will suitably comprise a fraction larger than 90 wt % which boils between 160 and 400° C., preferably to about 370° C. Examples of Fischer-Tropsch derived kerosene and gas oils are described in EP-A-0583836, WO-A-97/14768, WO-A-97/14769, WO-A-00/11116, WO-A-00/11117, WO-A-01/83406, WO-A-01/83648, WO-A-01/83647, WO-A-01/83641, WO-A-00/20535, WO-A-00/20534, EP-A-1101813, U.S. Pat. Nos. 5,766,274, 5,378,348, 5,888,376 and 6,204,426.

The Fischer-Tropsch product will suitably contain more than 80 wt % and more suitably more than 95 wt % iso and normal paraffins and less than 1 wt % aromatics, the balance being naphthenics compounds. The content of sulphur and nitrogen will be very low and normally below the detection limits for such compounds. For this reason the sulphur content of a fuel composition containing a Fischer-Tropsch product may be very low.

The fuel composition preferably contains no more than 5000 ppmw sulphur, more preferably no more than 500 ppmw, or no more than 350 ppmw, or no more than 150 ppmw, or no more than 100 ppmw, or no more than 50 ppmw, or most preferably no more than 10 ppmw sulphur.

In addition to the alkyl levulinates and the above-mentioned one or more additional components, each of which components contains one or more aromatic constituents, the fuel composition of the present invention may, if required, contain one or more additives as described below.

The base fuel may itself be additivated (additive-containing) or unadditivated (additive-free). If additivated, e.g. at the refinery, it will contain minor amounts of one or more additives selected for example from anti-static agents, pipeline drag reducers, flow improvers (e.g. ethylene/vinyl acetate copolymers or acrylate/maleic anhydride copolymers), lubricity additives, antioxidants and wax anti-settling agents.

Detergent-containing diesel fuel additives are known and commercially available. Such additives may be added to diesel fuels at levels intended to reduce, remove, or slow the build up of engine deposits.

Examples of detergents suitable for use in fuel additives for the present purpose include polyolefin substituted succinimides or succinamides of polyamines, for instance polyisobutylene succinimides or polyisobutylene amine succinamides, aliphatic amines, Mannich bases or amines and polyolefin (e.g. polyisobutylene) maleic anhydrides. Succinimide dispersant additives are described for example in GB-A-960493, EP-A-0147240, EP-A-0482253, EP-A-0613938, EP-A-0557516 and WO-A-98/42808. Particularly preferred are polyolefin substituted succinimides such as polyisobutylene succinimides.

The additive may contain other components in addition to the detergent. Examples are lubricity enhancers; dehazers, e.g. alkoxylated phenol formaldehyde polymers; anti-foaming agents (e.g. polyether-modified polysiloxanes); ignition improvers (cetane improvers) (e.g. 2-ethylhexyl nitrate (EHN), cyclohexyl nitrate, di-tert-butyl peroxide and those disclosed in U.S. Pat. No. 4,208,190 at column 2, line 27 to column 3, line 21); anti-rust agents (e.g. a propane-1,2-diol semi-ester of tetrapropenyl succinic acid, or polyhydric alcohol esters of a succinic acid derivative, the succinic acid derivative having on at least one of its alpha-carbon atoms an unsubstituted or substituted aliphatic hydrocarbon group containing from 20 to 500 carbon atoms, e.g. the pentaerythritol diester of polyisobutylene-substituted succinic acid); corrosion inhibitors; reodorants; anti-wear additives; anti-oxidants (e.g. phenolics such as 2,6-di-tert-butylphenol, or phenylenediamines such as N,N′-di-sec-butyl-p-phenylenediamine); metal deactivators; and combustion improvers.

It is particularly preferred that the additive include a lubricity enhancer, especially when the fuel composition has a low (e.g. 500 ppmw or less) sulphur content. In the additivated fuel composition, the lubricity enhancer is conveniently present at a concentration of less than 1000 ppmw, preferably between 50 and 1000 ppmw, more preferably between 100 and 1000 ppmw. Suitable commercially available lubricity enhancers include ester- and acid-based additives. Other lubricity enhancers are described in the patent literature, in particular in connection with their use in low sulphur content diesel fuels, for example in:

-   -   the paper by Danping Wei and H. A. Spikes, “The Lubricity of         Diesel Fuels”, Wear, III (1986) 217-235;     -   WO-A-95/33805—cold flow improvers to enhance lubricity of low         sulphur fuels;     -   WO-A-94/17160—certain esters of a carboxylic acid and an alcohol         wherein the acid has from 2 to 50 carbon atoms and the alcohol         has 1 or more carbon atoms, particularly glycerol monooleate and         di-isodecyl adipate, as fuel additives for wear reduction in a         diesel engine injection system;     -   U.S. Pat. No. 5,490,864—certain dithiophosphoric         diester-dialcohols as anti-wear lubricity additives for low         sulphur diesel fuels; and     -   WO-A-98/01516—certain alkyl aromatic compounds having at least         one carboxyl group attached to their aromatic nuclei, to confer         anti-wear lubricity effects particularly in low sulphur diesel         fuels.

It is also preferred that the additive contain an anti-foaming agent, more preferably in combination with an anti-rust agent and/or a corrosion inhibitor and/or a lubricity additive.

Unless otherwise stated, the (active matter) concentration of each such additional component in the additivated fuel composition is preferably up to 10000 ppmw, more preferably in the range from 0.1 to 1000 ppmw, advantageously from 0.1 to 300 ppmw, such as from 0.1 to 150 ppmw.

The (active matter) concentration of any dehazer in the fuel composition will preferably be in the range from 0.1 to 20 ppmw, more preferably from 1 to 15 ppmw, still more preferably from 1 to 10 ppmw, advantageously from 1 to 5 ppmw. The (active matter) concentration of any ignition improver present will preferably be 2600 ppmw or less, more preferably 2000 ppmw or less, conveniently from 300 to 1500 ppmw.

If desired, the additive components, as listed above, may be co-mixed, preferably together with suitable diluent(s), in an additive concentrate, and the additive concentrate may be dispersed into the fuel, in suitable quantity to result in a composition of the present invention.

In the case of a diesel fuel composition, for example, the additive will typically contain a detergent, optionally together with other components as described above, and a diesel fuel-compatible diluent, which may be a carrier oil (e.g. a mineral oil), a polyether, which may be capped or uncapped, a non-polar solvent such as toluene, xylene, white spirits and those sold by Shell companies under the trade mark “SHELLSOL”, and/or a polar solvent such as an ester and, in particular, an alcohol, e.g. hexanol, 2-ethylhexanol, decanol, isotridecanol and alcohol mixtures such as those sold by Shell companies under the trade mark “LINEVOL”, especially LINEVOL 79 alcohol which is a mixture of C₇₋₉ primary alcohols, or a C₁₂₋₁₄ alcohol mixture which is commercially available.

The total content of the additives may be suitably between 0 and 10000 ppmw and preferably below 5000 ppmw.

Preferably, the alkyl levulinate concentration in the fuel composition accords with one or more of the following parameters:

(i) at least 1% v; (ii) at least 2% v; (iii) at least 3% v; (iv) at least 4% v; (v) at least 5% v; (vi) up to 6% v; (vii) up to 8% v; (viii) up to 10% v, (xi) up to 12% v, (x) up to 35% v, with ranges having features (i) and (x), (ii) and (ix), (iii) and (viii), (iv) and (vii), and (v) and (vi) respectively being progressively more preferred.

In this specification, amounts (concentrations, % v, ppmw, wt %) of components are of active matter, i.e. exclusive of volatile solvents/diluent materials.

The present invention is particularly applicable where the fuel composition is used or intended to be used in a direct injection diesel engine, for example of the rotary pump, in-line pump, unit pump, electronic unit injector or common rail type, or in an indirect injection diesel engine. The fuel composition may be suitable for use in heavy and/or light duty diesel engines.

As mentioned above, it is also applicable where the fuel composition is used in heating applications, for example boilers. Such boilers include standard boilers, low temperature boilers and condensing boilers, and are typically used for heating water for commercial or domestic applications such as space heating and water heating.

The present invention may lead to any of a number of advantageous effects, including good engine low temperature performance.

The present invention will now be further described by reference to the following Examples, in which, unless otherwise indicated, parts and percentages are by weight, and temperatures are in degrees Celsius:

Fuels were blended with additives by adding said additives to base fuel at ambient temperature (20⁰° C.) and homogenising.

The following additives were used:

-   ethyl levulinate (available ex. Avocado); -   n-butyl levulinate (available ex. Aldrich); and -   n-pentyl levulinate (available ex. City Chemical or by the reaction     of 1-pentanol (available ex. Aldrich) with levulinic acid (available     ex. Aldrich)).

EXAMPLES

The miscibility of levulinates depends to some extent on base fuel properties. Two base fuels representative of the European market were chosen to explore this effect, i.e. (1) Fuel A was a Dreyfuss ULSD, a hydrotreated AGO having a cloud point of −27° C. and an aromatics content of 22% m; and (2) Fuel B was a Swedish Class 1 AGO, which is a low density, low aromatics (4% m) diesel fuel with a cloud point of −38° C. Both base fuels met the EN590 specification.

The properties of Fuels A and B are given in Table 1. TABLE 1 Fuel A Fuel B Density @ 15° C., 822 815 kg/m³ Distillation T50, 242 235 ° C. Distillation T95, 304 272 ° C. Cetane Number 54 54 Viscosity @40° C., 2.10 2.03 mm²/s Sulphur, mg/kg 10 <5 Cloud Point, ° C. −27 −38 Aromatics, % m 22 4

For screening purposes, a simple test method was used to determine the room temperature (20° C.) limit of miscibility of ethyl levulinate. Accurately metered volumes of ester were added sequentially to a known volume of diesel fuel in a 15 ml glass vial, shaken and observed. The first appearance of haze was recorded as the room temperature limit of miscibility for the mixture. The results are shown in Table 2 and clearly show that Fuel A, with the higher aromatic content, solubilised more ethyl levulinate than Fuel B. TABLE 2 Fuel A Fuel B 14% v 7% v

The miscibility of various alkyl levulinates was measured using a method based on the ASTM D2500 “Cloud Point” procedure. In this procedure, a sample of fuel (40 ml) is cooled from ambient temperature (20° C.) in a series of thermostat baths maintained at progressively lower temperatures. The sample is examined at 1° C. intervals as it cools to its wax cloud point. In addition to the wax cloud point temperature described in ASTM D2500, a further two temperatures were recorded coinciding with the following observations, if they occurred:

(1) the appearance of the first haze,

(2) the first sign of dropout of a separate liquid phase. In each case, cooling continued to the wax cloud point—beyond which, no further phase separation could be observed reliably because the sample became opaque.

Solutions of the esters ethyl levulinate, n-butyl levulinate and n-pentyl levulinate in Fuel A were blended at various concentrations and the miscibility of each blend was measured. The results are shown in Table 3 below. TABLE 3 Ester Phase separation temperature (° C.) concentration ethyl n-butyl n-pentyl (% v) levulinate levulinate levulinate 2 W W W 3 W W W 4 −17 W W 5 −10 W W 6  −5* W W 8    7 W W 10   14 W W W denotes that the mixture was cooled to the wax cloud point (−27° C. for Fuel A) without liquid separation; *= extrapolated value

The miscibility tests were repeated using Fuel B. The results are shown in Table 4. TABLE 4 Ester Phase separation temperature (° C.) concentration ethyl n-butyl n-pentyl (% v) levulinate levulinate levulinate 2 −26 W W 3 −10 W W 4    3 W W 5    5  −31* W 6    10* −26 W 8 — −22 −33 10 — −18 −28 W denotes that the mixture was cooled to the wax cloud point (−38° C. for Fuel B) without liquid separation; *= extrapolated value

It was found that, for each of Fuels A and B, when the aromatic content was increased by adding the solvent “SHELLSOL AB” (ex. Shell), the phase separation temperature was lowered throughout the range of concentrations of ethyl levulinate. The results of tests which demonstrate this effect are set out in Table 5: TABLE 5 Phase separation temperature (° C.) Fuel A plus Fuel B plus Fuel A SHELLSOL Fuel B SHELLSOL Ethyl (total AB (total (total AB (total levulinate aromatics aromatics aromatics aromatics (% v) 22% m) 40% m) 4% m) 20% m) 2 W — −26 −41 3 W <−30 −10  −30* 4 −17 <−30    3 −22 5 −10 <−30    5  −15* 6  −5* <−30    10*  −8 W denotes that the mixture was cooled to the wax cloud point (−27° C. for Fuel A) without liquid separation; *= extrapolated value 

1. A fuel composition comprising a gas oil base fuel, an alkyl levulinate and at least one additional component, each of which component contains one or more aromatic constituents.
 2. The fuel composition of claim 1 wherein said additional component is present in the amount of 1 to 50% m of the fuel composition.
 3. The fuel composition of claim 1 wherein the alkyl levulinate is selected from C₂₋₈ alkyl levulinates.
 4. The fuel composition of claim 3 wherein the alkyl levulinate is selected from the group consisting of ethyl levulinate, n-propyl levulinate, n-butyl levulinate, n-pentyl levulinate, 2-hexyl levulinate, 2-ethyl hexyl levulinate and mixtures thereof.
 5. The fuel composition of claim 4 wherein the alkyl levulinate is selected from ethyl levulinate, n-butyl levulinate and/or n-pentyl levulinate.
 6. The fuel composition of claim 1 wherein said additional component is selected from (i) refinery product streams with aromatics contents higher than that of the base fuel and (ii) aromatic solvents, boiling in the normal temperature range of gas oil.
 7. A method of reducing the phase separation temperature of a fuel composition containing a gas oil base fuel and an alkyl levulinate, said method comprising increasing the level of aromatic constituents in the fuel composition.
 8. A method of reducing the phase separation temperature of a fuel composition containing a gas oil base fuel and an alkyl levulinate, said method comprising the steps of incorporating in the fuel composition one or more additional components, each of which components contains one or more aromatic constituents.
 9. The method of claim 7 wherein said alkyl levulinate is selected from C₂₋₈ alkyl levulinates.
 10. The method of claim 8 wherein said alkyl levulinate is selected from C₂₋₈ alkyl levulinates.
 11. The method of clam 9 wherein said alkyl levulinate is selected from the group consisting of ethyl levulinate, n-propyl levulinate, n-butyl levulinate, n-pentyl levulinate, 2-hexyl levulinate, 2-ethyl hexyl levulinate and mixtures thereof.
 12. The method of clam 8 wherein said alkyl levulinate is selected from the group consisting of ethyl levulinate, n-propyl levulinate, n-butyl levulinate, n-pentyl levulinate, 2-hexyl levulinate, 2-ethyl hexyl levulinate and mixtures thereof.
 13. The method of claim 7 wherein said one or more additional components are selected from (i) refinery product streams with aromatics contents higher than that of the base fuel and (ii) aromatic solvents, boiling in the normal temperature range of gas oil.
 14. The method of claim 8 wherein said one or more additional components are selected from (i) refinery product streams with aromatics contents higher than that of the base fuel and (ii) aromatic solvents, boiling in the normal temperature range of gas oil.
 15. A method of operating a compression ignition engine and/or a vehicle which is powered by such an engine comprising the step of introducing into a combustion chamber of the engine a fuel composition of claim
 1. 16. A method of operating a compression ignition engine and/or a vehicle which is powered by such an engine comprising the step of introducing into a combustion chamber of the engine a fuel composition of claim
 2. 17. A method of operating a compression ignition engine and/or a vehicle which is powered by such an engine comprising the step of introducing into a combustion chamber of the engine a fuel composition of claim
 3. 18. A method of operating a heating appliance provided with a burner comprising the step of supplying to said burner a fuel composition of claim
 1. 19. A method of operating a heating appliance provided with a burner comprising the step of supplying to said burner a fuel composition of claim
 2. 20. A method of operating a heating appliance provided with a burner comprising the step of supplying to said burner a fuel composition of claim
 3. 21. A process for the preparation of a fuel composition comprising the step of blending a gas oil base fuel, an alkyl levulinate and at least one additional component, each of which component contains one or more aromatic constituents.
 22. The process of claim 21 wherein the additional component is present in the amount of 1 to 50% m of said fuel composition. 